The present invention relates generally to electrical circuits, and more particularly to a circuit and method of generating differential signals exhibiting phase accuracy at high frequencies.
Electrical circuits are utilized in a myriad of diverse applications, for example, computers, communication devices, industrial equipment, etc. In many of the applications which employ such circuits, differential signals are utilized to effectuate various functions. In addition, in many applications, the frequency of signals within such circuits are increasing in order to provide improved speed, conforming to communication protocols or standards, etc. In high frequency applications such as RF (radio frequency) communications, circuits employing differential type signals sometimes suffer from problems relating to phase delay. That is, one of the differential signals (e.g., RFout(+)) is not exactly 180 degrees out of the phase with the other corresponding differential signal (e.g., RFout(xe2x88x92)). Such phase imbalances may result in various undesirable effects.
One type of circuit system which sometimes utilizes differential signals is a communications receiver in a wireless application such as a cellular phone. An exemplary portion of a conventional heterodyne type receiver is illustrated in prior art FIG. 1. A heterodyne receiver translates the desired RF signal to one or more intermediate frequencies before demodulation. The receiver system is composed of several active and passive function blocks and each contributes to the system""s overall signal gain and noise figure (NF). The system 10 of FIG. 1 includes an antenna 12, a duplexer 14, an amplifier 16, one or more filters 18a and 18b, and a mixer 20 driven by a local oscillator 21 (LO).
The antenna 12 provides an interface between free space and the receiver input. The duplexer 14 interfaces with the antenna 12 and allows simultaneous transmitter and receiver operations with a single antenna. The duplexer 14 operates to isolate the receiver system 10 and a transmitter 22 from each other while providing a generally low loss connection to the antenna 12 for both systems.
The system 10 of FIG. 1 also includes the amplifier 16, typically a low-noise amplifier (LNA) that increases the amplitude of the signal received from the antenna 12 which allows for further processing by the receiver 10. An ideal amplifier increases the amplitude of the received signal without adding distortion or noise. Real world amplifiers, however, add noise and distortion to the received signal, and attempts are made to minimize signal degradations. The LNA 16 is the first amplifier after the antenna 12 in the system 10 and contributes most significantly to the system noise figure, consequently the amplifier 16 is typically designed to minimize noise, and hence the name LNA. The LNA 16 is typically constructed using active devices which operate in their linear range, however, the LNA output signal is not always perfectly linear, and distortion is added to the amplified signal due to nonlinearities of the one or more transistors therein.
The system 10 also includes one or more filters 18a and 18b, respectively. The filters form one or more networks which allow a range of RF frequencies to pass therethrough (oftentimes called bandpass filters). The filters block RF signals outside of their designed passband. When used, the RF filter 18a which is located before the LNA 16 is called a preselect filter and the post-amplifier RF filter 18b is often called the image-reject filter. The preselect filter 18a prevents signals far outside of the desired passband from saturating the front end and producing intermodulation distortion products related to those signals at far away frequencies, while the image-reject filter 18b rejects spurious response type signals. Lastly, the system 10 includes the mixer 20 which translates the received, filtered and amplified RF signal to both a higher and lower intermediate frequency (IF) value. One of the intermediate frequencies is passed while the other is rejected (e.g., called either up-conversion or down-conversion, respectively), using translation with the LO signal that mixes with the RF signal.
Many conventional mixers are designed to receive a differential input because differential signals help in decoupling the system 10 from noise in the integrated circuit substrate, thereby lowering the system NF, and aid in facilitating high device integration. Because the mixer 20 is designed to receive a differential signal input and the antenna 12 generates a single received signal, the system 10 must transform the single-ended signal into a differential signal somewhere between the antenna 12 and the mixer 20.
Conventional solutions which perform a transformation from a single-ended signal to differential signals before the LNA 16 have been found undesirable because prior to amplification the received signal is weak and the transformation results in too much loss, thereby degrading the integrity of the received signal. Similarly, conventional post-LNA transformation solutions have been found to be undesirable because of linearity issues. For example, a post-LNA solution sometimes utilizes a unity-type buffer coupled to the output of the LNA. Since the received signal exiting the LNA has been amplified (e.g., by about 20 dB), the post-LNA buffer must operate in a linear range for a substantially higher power signal, which results in an undesirable increase in power consumption. Since both pre-LNA and post-LNA single-to-differential transformation solutions are unsatisfactory, attempts have been made to integrate the transformation of a single RF signal to a differential signal within the LNA 16.
Therefore there is a need in the art for a circuit and method which provides a single-to-differential signal transformation functionality integrated within a low-noise amplifier or other type circuit arrangements such as buffers, etc.
According to the present invention, a circuit and method of transforming a single-ended signal to differential signals exhibiting good phase balance independent of signal frequency is disclosed.
According to one aspect of the present invention a circuit is disclosed which receives and single input signal and uses the signal to generate a pair of differential signals. The circuit includes a differential signal phase balance circuit that analyzes the phase of the differential signals and provides compensation based on the phase analysis in order to cause the differential signals to more closely be 180 degrees out of phase with one another independent of signal frequency. The present invention may be employed in various types of single-to-differential circuit applications, for example, buffers and amplifiers.
According to another aspect of the present invention, a single-to-differential LNA is disclosed which exhibits good phase balance independent of signal frequency. According to one exemplary aspect of the present invention, the LNA includes two coupled cascode type LNA amplifiers wherein an AC ground conventionally associated with a bias input is removed and a control node associated with the amplifier consequently is allowed to vary due to parasitic-type coupling effects. The control node voltage variations are a function of the phase balance of the differential signals and cause the timing at which various circuit functions occur to change. Such changes result in the phase of the differential signals to more closely be 180 degrees out of phase with one another, thus providing good phase balance.
According to yet another aspect of the present invention, a method of transforming a single-ended signal into a pair of differential signals exhibiting good phase balance independent of frequency is disclosed. The method includes generating the differential signals using the received single-ended signal and analyzing the phase of the differential signals. A signal compensation is then provided to one or both of the differential signals, wherein the compensation is a function of the degree to which the differential signals are imbalanced. The compensation causes the phase of the differential signals to more closely be 180 degrees out of phase.
According to still another aspect of the present invention, a method of providing good phase balance in a single-to-differential LNA is disclosed. The method includes coupling the differential signals (e.g., capacitive coupling) to a control node of the LNA. The coupling causes the control node voltage to vary based on the phase relationship of the differential signals and the control node voltage variations cause the functionality by which the signals are amplified to be altered and such alterations force the phase of the differential signals to more closely be 180 degrees out of phase with one another.
To the accomplishment of the foregoing and related ends, the invention, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such embodiments and their equivalents. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.